Autoacceleration Trommsdorff

Autoacceleration, Glass and Zutty (S) and Burnett and Melville 9) reported an increase in the rate and average degree of polymerization with increasing solution viscosity, heterogeneous conditions and chain coiling for free radical, vinyl polymerizations. Autoacceleration is also called Trommsdorff. (10) effect. 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

There is an increase in temperature as heat removal slows, along with a corresponding increase in the reaction rate. This phenomenon is known as the autoacceleration or Trommsdorff effect and can lead to catastrophic results if not properly controlled. Even with snccessfnl control of the reaction, it is difficult to remove the traces of remaining monomer from the polymer due to decreased diffusion. Similarly, it is difficult to get the reactions to proceed to completion due to limited monomer mobility. 

Frequently, even if as little as 20% of the monomer has polymerized, an autoaccelerating polymerization effect will take place. This may manifest itself in an increase in the heat evolved as the process nears completion. Particularly in large-scale, industrial polymerizations, this effect, known as the Trommsdorff effect or gel effect, may be quite dangerous. In fact, serious explosions have... 

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

Studies of the copolymerization of VDC with methyl acrylate (MA) over a composition range of 0—16 wt % showed that near the intermediate composition (8 wt %), the polymerization rates neady followed normal solution polymerization kinetics (49). However, at the two extremes (0 and 16 wt % MA), copolymerization showed significant autoacceleration. The observations are important because they show the significant complexities in these copolymerizations. The auto acceleration for the homopolymerization, ie, 0 wt % MA, is probably the result of a surface polymerization phenomenon. On the other hand, the auto acceleration for the 16 wt % MA copolymerization could be the result of Trommsdorff and Norrish-Smith effects. 

In free-radical polymerization, the autoacceleration (or gel effect, Trommsdorff effect) has been known for a long time 161 168>. 

For the major duration of a chain polymerisation the reaction is first-order in monomer concentration. However, at high conversions of monomer to polymer using either undiluted monomer or concentrated solutions there is a significant deviation from first-order kinetics. Under such circumstances the rate of reaction (and also molar mass of pol) mer) increases considerably. This so-called autoacceleration is sometimes referred to as the Trommsdorff-Norrish effect, after two of the pioneers in the study of polymerisation kinetics, who first noticed its occurrence. 

The major point of Interest in the polymerization curve, Figure 2, is the onset of the Trommsdorff effect, sometimes called autoacceleration. As is well known, the molecular weight increases rapidly after the onset of the Trommsdorff effect. 

A phenomenon that has a particularly significant effect in the value of and has been studied for many years is the so-called gel effect, which is also known as Trommsdorff or Norrish-Smith effect [14, 15], This consists in an autoacceleration of the reaction rate as the conversion increases, and it is due to an effective decrease in the termination rate as the growing radicals encounter more difficulty in diffusing in the increasingly viscous medium. As the value of decreases by several orders of magnitude in the course of the polymerization as a consequence of this phenomenon, the concentration of growing radicals [P] increases, as well as the polymerization rate (Eq. 4.7,... 

Free-radical polymerizations of certain monomers exhibit autoacceleration at high conversion via an additional mechanism, the isothermal gel effect or Trommsdorff effect (23-26). These reactions occur by the creation of a radical that attacks an unsaturated monomer, converting it to a radical, which can add to another monomer, propagating the chain. The chain growth terminates when two radical chains ends encounter each other, forming a stable... 

Both the Trommsdorff effect and thennal autocatalysis can lead to the autoacceleration in free-radical polymerization. The results indicated that the autoacceleration observed in the smallest test tube was mostly due to the Trommsdorff effect, while both the Trommsdorff effect and thermal autocatalysis strongly affected the onset of autoacceleration in the larger polymerization system. When an exothermic reaction is performed in a larger system, more heat tends to be accumulated in a reactor, since a sur ce-to-volume ratio is decreased as the size of the system increases. There was a critical size in the inner diameter of the test tube at which the behavior of the autoacceleration of the polymerization changes. 

The difTerence in the mechanism of autoacceleration, depending on the size of the test tubes, changed the character of the PMMA produced in the reaction system. As shown in Figure 3, both M and MJM increased in the smallest test tube after the onset of autoacceleration. This is a typical phenomenon for the Trommsdorff effect, since it is led by the retardation of the termination reaction. Increase of M of the polymer, however, was not observed in the larger test tubes. Thermal runaway led to rapid decomposition of the initiator to produce more radicals transiently. This process produced more polymers having a lower degree of polymerization, lowering M,. 

Polymerization of vinyl or methacrylic monomers (especially in conjunction with crosslinking monomers) within the wood often results in an autoacceleration during the latter phase of the polymerization this phenomenon is known as the Trommsdorff or gel effect in homopolymerization reactions (Duran and Meyer, 1972 Trommsdorff et a/., 1948). The gel effect arises from a decrease in the termination rate of the free radical polymerization, caused in turn by the effect of the local viscosity on the diffusion rates of the growing polymer chains. Since the heat of polymerization cannot be removed rapidly enough to maintain isothermal conditions, autoacceleration is characterized by a strong exotherm the intensity of the exotherm depends on the catalyst level, as illustrated in Figures 11.4 and 11.5 (Siau et al., 1968). 

The same applies to the Trommsdorff or autoacceleration effect frequently encountered in bulk polymerization processes. We will refer to some of these while discussing a number of aspects which are typically found with or are pronounced in... 

In the bulk polymerization of methyl methacrylate (MMA), the Trommsdorff effect sets in after an unperturbed period of about 15-20 % conversion. This enables a separation to be made between unperturbed and autoaccelerated kinetics. Benson and North, O Driscoll, Ito and others have attempted to describe the kinetics of autoacceleration or gelation by introducing a chain-length-dependent value of the termination rate constant... 

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